Proximal distal axis formation in the Drosophila leg: distinct functions of teashirt and homothorax in the proximal leg

Specification of the Drosophila Orcokinin A neurons by combinatorial coding

The central nervous system contains a daunting number of different cell types. Understanding how each cell acquires its fate remains a major challenge for neurobiology. The developing embryonic ventral nerve cord (VNC) of Drosophila melanogaster has been a powerful model system for unraveling the basic principles of cell fate specification. This pertains specifically to neuropeptide neurons, which typically are stereotypically generated in discrete subsets, allowing for unambiguous single-cell resolution in different genetic contexts. Here, we study the specification of the OrcoA-LA neurons, characterized by the expression of the neuropeptide Orcokinin A and located laterally in the A1-A5 abdominal segments of the VNC. We identified the progenitor neuroblast (NB; NB5-3) and the temporal window (castor/grainyhead) that generate the OrcoA-LA neurons. We also describe the role of the Ubx, abd-A, and Abd-B Hox genes in the segment-specific generation of these neurons. Additionally, our results indicate that the OrcoA-LA neurons are "Notch Off" cells, and neither programmed cell death nor the BMP pathway appears to be involved in their specification. Finally, we performed a targeted genetic screen of 485 genes known to be expressed in the CNS and identified nab, vg, and tsh as crucial determinists for OrcoA-LA neurons. This work provides a new neuropeptidergic model that will allow for addressing new questions related to neuronal specification mechanisms in the future.

A single ortholog of teashirt and tiptop regulates larval pigmentation and adult appendage patterning in Bombyx mori

Two paralogous genes, teashirt (tsh) and tiptop (tio), encode zinc-finger transcription factors and play important roles in insect growth and development. In the fruit fly, Drosophila melanogaster, tsh promotes trunk segmental identities and contributes to the patterning of other tissues during the embryonic stage. During the adult stage, tsh contributes to the specification and patterning of appendages, including the leg, wing and eye. While tio acts redundantly with tsh, flies lacking tio function are viable without deleterious phenotypes. This gene pair is present in the genomes of all Drosophila species but only as a single homologue in several other insect species. In Oncopeltus fasciatus and Tribolium castaneum, tsh/tio has been functionally characterized as specifying the identity of the leg during the adult stage. However, in lepidopteran insects which include large numbers of pests in agriculture and forestry, as well as the major silk producer silkworm Bombyx mori, the biological functions of tsh/tio are still poorly understood. In the current study, we performed functional analysis of tsh/tio by using both CRISPR/Cas9-mediated mutagenesis and transposon-mediated ectopic expression in B. mori. The results show that loss of tsh/tio function affected pigmentation during the larval stage and appendage pattering during the adult stage. RNA-seq analysis and subsequent q-RT-PCR analysis revealed that depletion of tsh/tio significantly elevated the expression of the kynurenine 3-monooxygenase gene, as well as melanin synthase-related genes during the larval stage. Furthermore, ubiquitous ectopic expression of tsh/tio induces developmental retardation and eventually larval lethality. These data reveal evolutionarily conserved functions of tsh/tio in controlling adult appendage patterning, as well as the novel function of regulating larval pigmentation in B. mori, providing novel insights into how tsh/tio regulates insect growth and development.

dTcf/Pangolin suppresses growth and tumor formation in Drosophila

Wnt/Wingless (Wg) signaling controls many aspects of animal development and is deregulated in different human cancers. The transcription factor dTcf/Pangolin (Pan) is the final effector of the Wg pathway in Drosophila and has a dual role in regulating the expression of Wg target genes. In the presence of Wg, dTcf/Pan interacts with β-catenin/Armadillo (Arm) and induces the transcription of Wg targets. In absence of Wg, dTcf/Pan partners with the transcriptional corepressor TLE/Groucho (Gro) and inhibits gene expression. Here, we use the wing imaginal disk of Drosophila as a model to examine the functions that dTcf/Pan plays in a proliferating epithelium. We report a function of dTcf/Pan in growth control and tumorigenesis. Our results show that dTcf/Pan can limit tissue growth in normal development and suppresses tumorigenesis in the context of oncogene up-regulation. We identify the conserved transcription factors Sox box protein 15 (Sox15) and Ftz transcription factor 1 (Ftz-f1) as genes controlled by dTcf/Pan involved in tumor development. In conclusion, this study reports a role for dTcf/Pan as a repressor of normal and oncogenic growth and identifies the genes inducing tumorigenesis downstream of dTcf/Pan.

Allocation of distinct organ fates from a precursor field requires a shift in expression and function of gene regulatory networks

A common occurrence in metazoan development is the rise of multiple tissues/organs from a single uniform precursor field. One example is the anterior forebrain of vertebrates, which produces the eyes, hypothalamus, diencephalon, and telencephalon. Another instance is the Drosophila wing disc, which generates the adult wing blade, the hinge, and the thorax. Gene regulatory networks (GRNs) that are comprised of signaling pathways and batteries of transcription factors parcel the undifferentiated field into discrete territories. This simple model is challenged by two observations. First, many GRN members that are thought to control the fate of one organ are actually expressed throughout the entire precursor field at earlier points in development. Second, each GRN can simultaneously promote one of the possible fates choices while repressing the other alternatives. It is therefore unclear how GRNs function to allocate tissue fates if their members are uniformly expressed and competing with each other within the same populations of cells. We address this paradigm by studying fate specification in the Drosophila eye-antennal disc. The disc, which begins its development as a homogeneous precursor field, produces a number of adult structures including the compound eyes, the ocelli, the antennae, the maxillary palps, and the surrounding head epidermis. Several selector genes that control the fates of the eye and antenna, respectively, are first expressed throughout the entire eye-antennal disc. We show that during early stages, these genes are tasked with promoting the growth of the entire field. Upon segregation to distinct territories within the disc, each GRN continues to promote growth while taking on the additional roles of promoting distinct primary fates and repressing alternate fates. The timing of both expression pattern restriction and expansion of functional duties is an elemental requirement for allocating fates within a single field.

A battery of transcription factors collectively called the retinal determination (RD) network controls the earliest steps in the specification of the fruit fly compound eye. Loss-of-function mutations lead to the loss of the compound eyes while over-expression of RD network members in non-retinal tissues induces the formation of ectopic eyes. These observations suggest that the network governs the growth, specification, and patterning of the eye field. Recent studies have also shown that the RD network represses the fates of the non-ocular tissues that are also derived from the disc such as the antenna, maxillary palp, and head epidermis. One inconsistency in the model for how this network controls eye specification is that many of its members are expressed throughout the entire eye-antennal disc. In this study, we show that early in development, the RD network is expressed throughout and promotes the growth of the entire eye-antennal disc. After the initial growth phase, the expression of these genes is restricted to just the eye field. This temporal and spatial limiting of the RD network to the developing eye is essential so that its role can expand to include promoting eye specification and repressing non-ocular fates.

The Hippo pathway effector Yki downregulates Wg signaling to promote retinal differentiation in the Drosophila eye

The evolutionarily conserved Hippo signaling pathway is known to regulate cell proliferation and maintain tissue homeostasis during development. We found that activation of Yorkie (Yki), the effector of the Hippo signaling pathway, causes separable effects on growth and differentiation of the Drosophila eye. We present evidence supporting a role for Yki in suppressing eye fate by downregulation of the core retinal determination genes. Other upstream regulators of the Hippo pathway mediate this effect of Yki on retinal differentiation. Here, we show that, in the developing eye, Yki can prevent retinal differentiation by blocking morphogenetic furrow (MF) progression and R8 specification. The inhibition of MF progression is due to ectopic induction of Wingless (Wg) signaling and Homothorax (Hth), the negative regulators of eye development. Modulating Wg signaling can modify Yki-mediated suppression of eye fate. Furthermore, ectopic Hth induction due to Yki activation in the eye is dependent on Wg. Last, using Cut (Ct), a marker for the antennal fate, we show that suppression of eye fate by hyperactivation of yki does not change the cell fate (from eye to antenna-specific fate). In summary, we provide the genetic mechanism by which yki plays a role in cell fate specification and differentiation - a novel aspect of Yki function that is emerging from multiple model organisms.

Wg and Wnt4 provide long-range directional input to planar cell polarity orientation in Drosophila

Planar cell polarity (PCP) is cellular polarity within the plane of an epithelial tissue or organ. PCP is established through interactions of the core Frizzled(Fz)/PCP factors and although their molecular interactions are beginning to be understood, the upstream input providing directional bias/polarity axis remains unknown. Among core PCP genes, Fz is unique as it regulates PCP both cell-autonomously and non-autonomously, with the extra-cellular domain of Fz acting as a ligand for Van-Gogh (Vang). We demonstrate in Drosophila wings that Wg and dWnt4 provide instructive regulatory input for PCP axis determination, establishing polarity axes along their graded distribution and perpendicular to their expression domain borders. Loss-of-function studies reveal that Wg/dWnt4 act redundantly in PCP determination. They affect PCP by modulating the intercellular interaction between Fz and Vang, which is thought to be a key step in setting up initial polarity, thus providing directionality to the PCP process.

Homothorax plays autonomous and nonautonomous roles in proximodistal axis formation and migration of the Drosophila renal tubules

The Drosophila Malpighian tubules (MpTs) serve as a functional equivalent of the mammalian renal tubules. The MpTs are composed of two pairs of epithelial tubes that bud from the midgut-hindgut boundary during embryogenesis. The MpT primordia grow, elongate and migrate through the body cavity to assume their final position and shape. The stereotypic pattern of MpT migration is regulated by multiple intrinsic and extrinsic signals, many of which are still obscure. In this work, we implicate the TALE-class homeoprotein Homothorax (Hth) in MpT patterning. We show that in the absence of Hth the tubules fail to rearrange and migrate. Hth plays both autonomous and nonautonomous roles in this developmental process. Within the tubules Hth is required for convergent extension and for defining distal versus proximal cell identities. The difference between distal and proximal cell identities seems to be required for proper formation of the leading loop. Outside the tubules, wide-range mesodermal expression of Hth is required for directing anterior migration. The nonautonomous effects of Hth on MpT migration can be partially attributed to its effects on homeotic determination along the anterior posterior axis of the embryo and to its effects on stellate cell (SC) incorporation into the MpT.

Cell-type-specific profiling of gene expression and chromatin binding without cell isolation: assaying RNA Pol II occupancy in neural stem cells

Cell-type-specific transcriptional profiling often requires the isolation of specific cell types from complex tissues. We have developed “TaDa,” a technique that enables cell-specific profiling without cell isolation. TaDa permits genome-wide profiling of DNA- or chromatin-binding proteins without cell sorting, fixation, or affinity purification. The method is simple, sensitive, highly reproducible, and transferable to any model system. We show that TaDa can be used to identify transcribed genes in a cell-type-specific manner with considerable temporal precision, enabling the identification of differential gene expression between neuroblasts and the neuroepithelial cells from which they derive. We profile the genome-wide binding of RNA polymerase II in these adjacent, clonally related stem cells within intact Drosophila brains. Our data reveal expression of specific metabolic genes in neuroepithelial cells, but not in neuroblasts, and highlight gene regulatory networks that may pattern neural stem cell fates.

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TaDa is a method for cell-type-specific profiling of chromatin binding proteins

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TaDa does not require cell sorting, fixation, or affinity purification

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This is a highly sensitive and robust technique for transcriptional profiling

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We report differential RNA Pol II binding in clonally related stem cells

The tiptop/teashirt genes regulate cell differentiation and renal physiology in Drosophila

The physiological activities of organs are underpinned by an interplay between the distinct cell types they contain. However, little is known about the genetic control of patterned cell differentiation during organ development. We show that the conserved Teashirt transcription factors are decisive for the differentiation of a subset of secretory cells, stellate cells, in Drosophila melanogaster renal tubules. Teashirt controls the expression of the water channel Drip, the chloride conductance channel CLC-a and the Leukokinin receptor (LKR), all of which characterise differentiated stellate cells and are required for primary urine production and responsiveness to diuretic stimuli. Teashirt also controls a dramatic transformation in cell morphology, from cuboidal to the eponymous stellate shape, during metamorphosis. teashirt interacts with cut, which encodes a transcription factor that underlies the differentiation of the primary, principal secretory cells, establishing a reciprocal negative-feedback loop that ensures the full differentiation of both cell types. Loss of teashirt leads to ineffective urine production, failure of homeostasis and premature lethality. Stellate cell-specific expression of the teashirt paralogue tiptop, which is not normally expressed in larval or adult stellate cells, almost completely rescues teashirt loss of expression from stellate cells. We demonstrate conservation in the expression of the family of tiptop/teashirt genes in lower insects and establish conservation in the targets of Teashirt transcription factors in mouse embryonic kidney.

New components of Drosophila leg development identified through genome wide association studies

The adult Drosophila melanogaster body develops from imaginal discs, groups of cells set-aside during embryogenesis and expanded in number during larval stages. Specification and development of Drosophila imaginal discs have been studied for many years as models of morphogenesis. These studies are often based on mutations with large developmental effects, mutations that are often lethal in embryos when homozygous. Such forward genetic screens can be limited by factors such as early lethality and genetic redundancy. To identify additional genes and genetic pathways involved in leg imaginal disc development, we employed a Genome Wide Association Study utilizing the natural genetic variation in leg proportionality found in the Drosophila Genetic Reference Panel fly lines. In addition to identifying genes already known to be involved in leg development, we identified several genes involved in pathways that had not previously been linked with leg development. Several of the genes appear to be involved in signaling activities, while others have no known roles at this time. Many of these uncharacterized genes are conserved in mammals, so we can now begin to place these genes into developmental contexts. Interestingly, we identified five genes which, when their function is reduced by RNAi, cause an antenna-to-leg transformation. Our results demonstrate the utility of this approach, integrating the tools of quantitative and molecular genetics to study developmental processes, and provide new insights into the pathways and networks involved in Drosophila leg development.

Homeotic functions of the Teashirt transcription factor during adult Drosophila development

During Drosophila development region-specific regulation of target genes by Hox proteins is modulated by genetic interactions with various cofactors and genetic collaborators. During embryogenesis one such modulator of Hox target specificity is the zinc-finger transcription factor Teashirt (Tsh) that is expressed in the developing trunk and cooperatively functions with trunk-specific Hox proteins to promote appropriate segment fate. This embryonic function of Tsh is characterized as homeotic since loss of embryonic Tsh activity leads to transformation of trunk segments toward head identity. In addition to this embryonic homeotic role, Tsh also performs vital Hox-independent functions through patterning numerous embryonic, larval and adult structures. Here we address whether the homeotic function of Tsh is maintained throughout development by investigating its contribution to patterning the adult abdomen. We show that Tsh is expressed throughout the developing abdomen and that this expression is dependent on the three Bithorax Hox proteins Ultrabithorax, Abdominal-A and Abdominal-B. Conditional reduction of Tsh activity during pupation reveals broad homeotic roles for this transcription factor throughout the adult abdomen. Additionally we show that, as during embryogenesis, the tsh paralog tiptop (tio) plays a partially redundant role in this homeotic activity.

A dynamic network of morphogens and transcription factors patterns the fly leg

Animal appendages require a proximodistal (PD) axis, which forms orthogonally from the two main body axes, anteroposterior and dorsoventral. In this review, we discuss recent advances that begin to provide insights into the molecular mechanisms controlling PD axis formation in the Drosophila leg. In this case, two morphogens, Wingless (Wg) and Decapentaplegic (Dpp), initiate a genetic cascade that, together with growth of the leg imaginal disc, establishes the PD axis. The analysis of cis-regulatory modules (CRMs) that control the expression of genes at different positions along the PD axis has been particularly valuable in dissecting this complex process. From these experiments, it appears that only one concentration of Wg and Dpp are required to initiate PD axis formation by inducing the expression of Distal-less (Dll), a homeodomain-encoding gene that is required for leg development. Once Dll is turned on, it activates the medially expressed gene dachshund (dac). Cross-regulation between Dll and dac, together with cell proliferation in the growing leg imaginal disc, results in the formation of a rudimentary PD axis. Wg and Dpp also initiate the expression of ligands for the EGFR pathway, which in turn induces the expression of a series of target genes that pattern the distal-most portion of the leg.

A dissection of the teashirt and tiptop genes reveals a novel mechanism for regulating transcription factor activity

In the Drosophila eye the retinal determination (RD) network controls both tissue specification and cell proliferation. Mutations in network members result in severe reductions in the size of the eye primordium and the transformation of the eye field into head cuticle. The zinc-finger transcription factor Teashirt (Tsh) plays a role in promoting cell proliferation in the anterior most portions of the eye field as well as in inducing ectopic eye formation in forced expression assays. Tiptop (Tio) is a recently discovered paralog of Tsh. It is distributed in an identical pattern to Tsh within the retina and can also promote ectopic eye development. In a previous study we demonstrated that Tio can induce ectopic eye formation in a broader range of cell populations than Tsh and is also a more potent inducer of cell proliferation. Here we have focused on understanding the molecular and biochemical basis that underlies these differences. The two paralogs are structurally similar but differ in one significant aspect: Tsh contains three zinc finger motifs while Tio has four such domains. We used a series of deletion and chimeric proteins to identify the zinc finger domains that are selectively used for either promoting cell proliferation or inducing eye formation. Our results indicate that for both proteins the second zinc finger is essential to the proper functioning of the protein while the remaining zinc finger domains appear to contribute but are not absolutely required. Interestingly, these domains antagonize each other to balance the overall activity of the protein. This appears to be a novel internal mechanism for regulating the activity of a transcription factor. We also demonstrate that both Tsh and Tio bind to C-terminal Binding Protein (CtBP) and that this interaction is important for promoting both cell proliferation and eye development. And finally we report that the physical interaction that has been described for Tsh and Homothorax (Hth) do not occur through the zinc finger domains.

Regulation of ocellar specification and size by twin of eyeless and homothorax

The retinal determination gene network (RDGN) constitutes a paradigm of a gene network controlling organ specification and growth. In this study, we probed the RDGN in the Drosophila ocelli, a set of simple eyes located on the fly's dorsal head, by studying the expression, regulation, and function of toy, hth, eya, and so, members of the Pax6, Meis, Eya, and Six gene families. Our results highlight the role of the pax6 gene toy, together with the hh signaling pathway, in the initiation of eya and so expression; the engagement of eya and so in a feedback loop necessary for their full expression; and the interplay between hh signaling and hth as a mechanism of organ size control, as general regulatory steps in the specification of visual organs.

Teashirt-3, a novel regulator of muscle differentiation, associates with BRG1-associated factor 57 (BAF57) to inhibit myogenin gene expression

In adult muscles and under normal physiological conditions, satellite cells are found in a quiescent state but can be induced to enter the cell cycle by signals resulting from exercise, injury-induced muscle regeneration, or specific disease states. Once activated, satellite cells proliferate, self-renew, and differentiate to form myofibers. In the present study, we found that the zinc finger-containing factor Teashirt-3 (TSHZ3) was expressed in quiescent satellite cells of adult mouse skeletal muscles. We showed that following treatment with cardiotoxin TSHZ3 was strongly expressed in satellite cells of regenerating muscles. Moreover, immunohistochemical analysis indicated that TSHZ3 was expressed in both quiescent and activated satellite cells on intact myofibers in culture. TSHZ3 expression was maintained in myoblasts but disappeared with myotube formation. In C2C12 myoblasts, we showed that overexpression of Tshz3 impaired myogenic differentiation and promoted the down-regulation of myogenin (Myog) and up-regulation of paired-box factor 7 (Pax7). Moreover, knockdown experiments revealed a selective effect of Tshz3 on Myog regulation, and transcriptional reporter experiments indicated that TSHZ3 repressed Myog promoter. We identified the BRG1-associated factor 57 (BAF57), a subunit of the SWI/SNF complex, as a partner of TSHZ3. We showed that TSHZ3 cooperated with BAF57 to repress MYOD-dependent Myog expression. These results suggest a novel mechanism for transcriptional repression by TSHZ3 in which TSHZ3 and BAF57 cooperate to modulate MyoD activity on the Myog promoter to regulate skeletal muscle differentiation.

Phylogeny of the teashirt-related zinc finger (tshz) gene family and analysis of the developmental expression of tshz2 and tshz3b in the zebrafish

The tshz genes comprise a family of evolutionarily conserved transcription factors. However, despite the major role played by Drosophila tsh during the development of the fruit fly, the expression and function of other tshz genes have been analyzed in a very limited set of organisms and, therefore, our current knowledge of these genes is still fragmentary. In this study, we perform detailed phylogenetic analyses of the tshz genes, identify the members of this gene family in zebrafish and describe the developmental expressions of two of them, tshz2 and tshz3b, and compare them with meis1, meis2.1, meis2.2, pax6a, and pax6b expression patterns. The expression patterns of these genes define a complex set of coexpression domains in the developing zebrafish brain where their gene products have the potential to interact.

Control of the spineless antennal enhancer: direct repression of antennal target genes by Antennapedia

It is currently thought that antennal target genes are activated in Drosophila by the combined action of Distal-less, homothorax, and extradenticle, and that the Hox gene Antennapedia prevents activation of antennal genes in the leg by repressing homothorax. To test these ideas, we analyze a 62bp enhancer from the antennal gene spineless that is specific for the third antennal segment. This enhancer is activated by a tripartite complex of Distal-less, Homothorax, and Extradenticle. Surprisingly, Antennapedia represses the enhancer directly, at least in part by competing with Distal-less for binding. We show that Antennapedia is required in the leg only within a proximal ring that coexpresses Distal-less, Homothorax and Extradenticle. We conclude that the function of Antennapedia in the leg is not to repress homothorax, as has been suggested, but to directly repress spineless and other antennal genes that would otherwise be activated within this ring.

Drosophila laminins act as key regulators of basement membrane assembly and morphogenesis

Laminins are heterotrimeric molecules found in all basement membranes. In mammals, they have been involved in diverse developmental processes, from gastrulation to tissue maintenance. The Drosophila genome encodes two laminin alpha chains, one beta and one Gamma, which form two distinct laminin trimers. So far, only mutations affecting one or other trimer have been analysed. In order to study embryonic development in the complete absence of laminins, we mutated the gene encoding the sole laminin beta chain in Drosophila, LanB1, so that no trimers can be made. We show that LanB1 mutant embryos develop until the end of embryogenesis. Electron microscopy analysis of mutant embryos reveals that the basement membranes are absent and the remaining extracellular material appears disorganised and diffuse. Accordingly, abnormal accumulation of major basement membrane components, such as Collagen IV and Perlecan, is observed in mutant tissues. In addition, we show that elimination of LanB1 prevents the normal morphogenesis of most organs and tissues, including the gut, trachea, muscles and nervous system. In spite of the above structural roles for laminins, our results unravel novel functions in cell adhesion, migration and rearrangement. We propose that while an early function of laminins in gastrulation is not conserved in Drosophila and mammals, their function in basement membrane assembly and organogenesis seems to be maintained throughout evolution.

Transcription factor choice in the Hippo signaling pathway: homothorax and yorkie regulation of the microRNA bantam in the progenitor domain of the Drosophila eye imaginal disc

The accurate control of cell proliferation and survival is critical for animal development. The Hippo tumor suppressor pathway regulates both of these parameters by controlling the nuclear availability of the transcriptional coactivator Yorkie (Yki), which regulates downstream target genes together with Scalloped (Sd), a DNA-binding protein. Here we provide evidence that Yki can also regulate target genes in conjunction with Homothorax (Hth) and Teashirt (Tsh), two DNA-binding transcription factors expressed in the uncommitted progenitor cells of the Drosophila eye imaginal disc. Clonal analyses demonstrate that Hth and Tsh promote cell proliferation and protect eye progenitor cells from apoptosis. Genetic epistasis experiments suggest that Hth and Tsh execute these functions with Yki, in part by up-regulating the microRNA bantam. A physical interaction between Hth and Yki can be detected in cell culture, and we show that Hth and Yki are bound to a DNA sequence approximately 14 kb upstream of the bantam hairpin in eye imaginal disc cells, arguing that this regulation is direct. These data suggest that the Hippo pathway uses different DNA-binding transcription factors depending on the cellular context. In the eye disc, Hth and Tsh provide spatial information to this pathway, promoting cell proliferation and survival in the progenitor domain.

Wings, horns, and butterfly eyespots: how do complex traits evolve?

Do novel complex traits evolve when pre-existent complex developmental networks are recruited into new places in the body? Here we look closely at the genomic fingerprints that are produced as a result of gene network co-option.

Zinc-finger paralogues tsh and tio are functionally equivalent during imaginal development in Drosophila and maintain their expression levels through auto- and cross-negative feedback loops

teashirt (tsh) and tiptop (tio) are two Drosophila gene paralogues encoding zinc-finger transcription factors. While tsh is an important developmental regulator, tio null individuals are viable and fertile. Here, we show that tio and tsh have coincident expression domains in the imaginal discs, the precursors of the adult body, and that both genes show similar functional properties when expressed ectopically. Furthermore, tio is able to rescue the development of tsh mutants, indicating that both genes are functionally equivalent during imaginal development. Of interest, the transcriptional regulation of tio and tsh is linked by a negative feedback loop. This mechanism might be required to maintain a tight control on the total levels of tio/tsh and could help explaining why Drosophila keeps an apparently dispensable gene.

Temporal and spatial windows delimit activation of the outer ring of wingless in the Drosophila wing

Extracellular signalling molecules play many roles in the development of higher organisms. They are used reiteratively in different tissues and stages, but the response of the receiving cells is controlled in a context dependent manner. The pattern of expression of the signalling molecule Wingless/WNT in Drosophila is extraordinarily complex. We have studied the mechanism that controls its expression and function in the outer ring of the Drosophila wing hinge. Our findings indicate that wingless expression is controlled by a dual mechanism: its initial activation requires the product of zinc finger homeodomain 2 and is subsequently repressed by the product of the gene complex elbow/no ocelli. This tight regulation restricts the activation of wingless temporally and spatially. Later in development, wingless expression is maintained by an autoregulatory loop that involves the product of homothorax. We have analyzed the phenotype of a wingless allelic combination that specifically removes the outer ring, and our results show that Wingless is required to promote local proliferation of the wing base cells. Thus, cell proliferation in the proximal-distal axis is controlled by the sequential activation of wingless in the inner ring and the outer ring at different stages of development.

SoxF is part of a novel negative-feedback loop in the wingless pathway that controls proliferation in the Drosophila wing disc

Wnt molecules act as mitogenic signals during the development of multiple organs, and the aberrant activity of their pathway is often associated with cancer. Therefore, the production of Wnts and the activity of their signaling pathway must be tightly regulated. We have investigated the mechanisms of this regulation in the Drosophila hinge, a domain within the wing imaginal disc that depends on the fly Wnt1 ortholog wingless (wg) for its proliferation. Our results uncover a new feedback loop in the wg pathway in which the spatially restricted activation of the Sox gene SoxF (Sox15) by wg represses its own transcription, thus ensuring tight regulation of growth control. rotund, a wing proximodistal patterning gene, excludes SoxF from a thin rim of cells. These cells are thus allowed to express wg and act as the source of mitogenic signal. This novel mode of action of a Sox gene on the Wnt pathway -- through transcriptional repression of a Wnt gene -- might be relevant to human disease, as loss of human SoxF genes has been implicated in colon carcinoma.

The origins of the Drosophila leg revealed by the cis-regulatory architecture of the Distalless gene

Limb development requires the elaboration of a proximodistal (PD) axis, which forms orthogonally to previously defined dorsoventral (DV) and anteroposterior (AP) axes. In arthropods, the PD axis of the adult leg is subdivided into two broad domains, a proximal coxopodite and a distal telopodite. We show that the progressive subdivision of the PD axis into these two domains occurs during embryogenesis and is reflected in the cis-regulatory architecture of the Distalless (Dll) gene. Early Dll expression, governed by the Dll304 enhancer, is in cells that can give rise to both domains of the leg as well as to the entire dorsal (wing) appendage. A few hours after Dll304 is activated, the activity of this enhancer fades, and two later-acting enhancers assume control over Dll expression. The LT enhancer is expressed in cells that will give rise to the entire telopodite, and only the telopodite. By contrast, cells that activate the DKO enhancer will give rise to a leg-associated larval sensory structure known as the Keilin's organ (KO). Cells that activate neither LT nor DKO, but had activated Dll304, will give rise to the coxopodite. In addition, we describe the trans-acting signals controlling the LT and DKO enhancers, and show, surprisingly, that the coxopodite progenitors begin to proliferate approximately 24 hours earlier than the telopodite progenitors. Together, these findings provide a complete and high-resolution fate map of the Drosophila appendage primordia, linking the primary domains to specific cis-regulatory elements in Dll.

Retinal determination genes as targets and possible effectors of extracellular signals

Retinal determination genes are sufficient to specify eyes in ectopic locations, raising the question of how these master regulatory genes define an eye developmental field. Genetic mosaic studies establish that expression of the retinal determination genes eyeless, teashirt, homothorax, eyes absent, sine oculis, and dachshund are each regulated by combinations of Dpp, Hh, N, Wg, and Ras signals in Drosophila. Dpp and Hh control eyeless, teashirt, sine oculis, and dachshund expression, Dpp and Ras control homothorax, and all the signaling pathways affect eyes absent expression. These results suggest that eye-specific development uses retinal determination gene expression to relay positional information to eye target genes, because the distinct, overlapping patterns of retinal determination gene expression reflect the activities of the extracellular signaling pathways.

Notch-mediated segmentation of the appendages is a molecular phylotypic trait of the arthropods

Arthropod limbs are arguably the most diverse organs in the animal kingdom. Morphological diversity of the limbs is largely based on their segmentation, because this divides the limbs into modules that can evolve separately for new morphologies and functions. Limb segmentation also distinguishes the arthropods from related phyla (e.g. onychophorans) and thus forms an important evolutionary innovation in arthropods. Understanding the genetic basis of limb segmentation in arthropods can thus shed light onto the mechanisms of macroevolution and the origin of a character (articulated limbs) that defines a new phylum (arthropods). In the fly Drosophila limb segmentation and limb growth are controlled by the Notch signaling pathway. Here we show that the Notch pathway also controls limb segmentation and growth in the spider Cupiennius salei, a representative of the most basally branching arthropod group Chelicerata, and thus this function must trace from the last common ancestor of all arthropods. The similarities of Notch and Serrate function between Drosophila and Cupiennius are extensive and also extend to target genes like odd-skipped, nubbin, AP-2 and hairy related genes. Our data confirm that the jointed appendages, which are a morphological phylotypic trait of the arthropods and the basis for naming the phylum, have a common developmental genetic basis. Notch-mediated limb segmentation is thus a molecular phylotypic trait of the arthropods.

The Drosophila gap gene giant has an anterior segment identity function mediated through disconnected and teashirt

The C2H2 zinc-finger-containing transcription factors encoded by the disconnected (disco) and teashirt (tsh) genes contribute to the regionalization of the Drosophila embryo by establishing fields in which specific Homeotic complex (Hom-C) proteins can function. In Drosophila embryos, disco and the paralogous disco-related (disco-r) are expressed throughout most of the epidermis of the head segments, but only in small patches in the trunk segments. Conversely, tsh is expressed extensively in the trunk segments, with little or no accumulation in the head segments. Little is known about the regulation of these genes; for example, what limits their expression to these domains? Here, we report the regulatory effects of gap genes on the spatial expression of disco, disco-r, and tsh during Drosophila embryogenesis. The data shed new light on how mutations in giant (gt) affect patterning within the anterior gt domain, demonstrating homeotic function in this domain. However, the homeosis does not occur through altered expression of the Hom-C genes but through changes in the regulation of disco and tsh.

Reciprocal roles for bowl and lines in specifying the peripodial epithelium and the disc proper of the Drosophila wing primordium

Central to embryonic development is the generation of molecular asymmetries across fields of undifferentiated cells. The Drosophila wing imaginal disc provides a powerful system with which to understand how such asymmetries are generated and how they contribute to formation of a complex structure. Early in development, the wing primordium is subdivided into a thin layer of peripodial epithelium (PE) and an apposing thickened layer of pseudostratified columnar epithelium (CE), known as the disc proper (DP). The DP gives rise to the wing blade, hinge and dorsal mesothorax, whereas the PE makes only a minor contribution to the ventral hinge and pleura. The mechanisms that generate this major asymmetry and its contribution to wing development are poorly understood. The Lines protein destabilizes the nuclear protein Bowl in ectodermal structures. Here, we show that Bowl accumulates in the PE from early stages of wing development and is absent from the DP. Broad inhibition of Bowl in the PE resulted in the replacement of the PE with a mirror image duplication of the DP. The failure to generate the PE severely compromised wing growth and the formation of the notum. Conversely, the activation of bowl in the DP (by removal or inhibition of lines function) resulted in the transformation of the DP into PE. Thus, we provide evidence that bowl and lines act as a binary switch to subdivide the wing primordium into PE and DP, and assign crucial roles for this asymmetry in wing growth and patterning.

Evidence for a novel role for dachshund in patterning the proximal arthropod leg

The branchiopod crustacean Triops longicaudatus has paddlelike thoracic appendages with few joints and multiple marginal lobes. Here, we explore the degree to which the Triops limb is patterned by the same network of genes known to pattern the uniramous, multi-jointed insect appendage. Insect leg patterning proceeds through a process of subdividing the leg into proximal, intermediate, and distal regions by the activity of the transcription factors hth/exd, dac, and Dll. The immature Triops limb is subdivided into large, discrete regional domains (proximal and distal) as defined by nuclear-EXD and DLL. We show that HTH expression in Triops overlaps cell-to-cell with n-EXD expression. In addition, dac is expressed in two domains: (1) adjacent to and partially overlapping the distal Dll domain and (2) along the medial margin of the developing leg. The DAC domain adjacent to the distal Dll domain supports the early establishment of the expected intermediate domain of DAC expression. The medial expression domain resolves over time into a series of reiterated stripes located on the lower side of each medial lobe. Later, this expression pattern correlates with the sclerotized regions associated with limb flexion. We propose that these stripes of DAC expression play a role in forming reiterated medial lobes. Unlike Drosophila, where the proximal distal patterning of the leg is coincident with patterning of reiterated structures (segments), we hypothesize that the patterning in Triops may reflect an ancestral state where the patterning of reiterated medial structures was not coincident with proximodistal limb patterning.

The Drosophila gene zfh2 is required to establish proximal-distal domains in the wing disc

Three main events characterize the development of the proximal-distal axis of the Drosophila wing disc: first, generation of nested circular domains defined by different combinations of gene expression; second, activation of wingless (wg) gene expression in a ring of cells; and third, an increase of cell number in each domain in response to Wg. The mechanisms by which these domains of gene expression are established and maintained are unknown. We have analyzed the role of the gene zinc finger homeodomain 2 (zfh2). We report that in discs lacking zfh2 the limits of the expression domains of the genes tsh, nub, rn, dve and nab coincide, and expression of wg in the wing hinge, is lost. We show that zfh2 expression is delimited distally by Vg, Nub and Dpp signalling, and proximally by Tsh and Dpp. Distal repression of zfh2 permits activation of nab in the wing blade and wg in the wing hinge. We suggest that the proximal-most wing fate, the hinge, is specified first and that later repression of zfh2 permits specification of the distal-most fate, the wing blade. We propose that proximal-distal axis development is achieved by a combination of two strategies: on one hand a process involving proximal to distal specification, with the wing hinge specified first followed later by the distal wing blade; on the other hand, early specification of the proximal-distal domains by different combinations of gene expression. The results we present here indicate that Zfh2 plays a critical role in both processes.

Expression patterns of the three Teashirt-related genes define specific boundaries in the developing and postnatal mouse forebrain

We compare the expression patterns of the three mouse Teashirt (mTsh) genes during development of the forebrain and at a postnatal stage. During development, mTsh genes are expressed in domains that are restricted both dorsoventrally and rostrocaudally, with major changes in expression level coinciding with compartment boundaries. Striking complementarities in the distribution of mTsh transcripts were observed in the developing diencephalon, telencephalon, and olfactory bulb (OB). A mTsh1-positive cell population is part of the DLX-positive population localized in the dorsalmost portion of the lateral ganglionic eminence (dLGE). Comparison of the mTsh1 expression domain with the domains of Er81 and Islet1, which mark two distinct progenitor populations in the subventricular zone of the LGE, suggests that mTsh1 marks OB interneuron progenitors. Furthermore, the distinct expression patterns of mTsh1 and mTsh2 in the ventral LGE and the dLGE highlight the differential contributions of these structures to the striatum and the amydaloid complex. For Sey/Sey mutants, we show that Pax6 function is critical for the correct specification of the mTsh1+ population in the dLGE during embryogenesis. At postnatal stages in the OB, mTsh1 is expressed in granule and periglomerular cells, which originate from the subpallium during development. Furthermore, mTsh1+ cells line the walls of the anterior lateral ventricle, a region that gives rise to the interneurons that migrate in the rostral migratory streams and populate the OB postnatally. Our results suggest a role for mTsh genes in the establishment of regional identity and specification of cell types in the developing and adult forebrain.

Frizzled-Dishevelled signaling specificity outcome can be modulated by Diego in Drosophila

Members of the Frizzled (Fz) family of seven-pass transmembrane receptors are required for the transduction of both Wnt-Fz/beta-catenin and Fz/planar cell polarity (PCP) signals. Although both pathways transduce signals via interactions between Fz and the cytoplasmic protein Dishevelled (Dsh), each pathway has specific and distinct effectors. One explanation for the pathway specificity is that signal-induced conformational changes result in unique Fz-Dsh interactions. Our mutational analyses of Fz-Dsh activities in vivo do however not support this model, since both pathways are affected by all mutations tested. Alternatively, the interaction of Fz or Dsh with other proteins could modulate the signaling outcome. We examined the role of a Dsh-binding PCP molecule, Diego (Dgo), in both Wnt-Fz/beta-catenin and Fz/PCP signaling. Both loss-of-function and gain-of-function results suggest that Dgo promotes Fz-Dsh/PCP signaling at the expense of Wnt-Fz/beta-catenin signaling. Our data suggest that Dgo sequesters Dsh to a functionally distinct Fz/PCP signaling compartment within the cell.

The appendage role of insect disco genes and possible implications on the evolution of the maggot larval form

Though initially identified as necessary for neural migration, Disconnected and its partially redundant paralog, Disco-related, are required for proper head segment identity during Drosophila embryogenesis. Here, we present evidence that these genes are also required for proper ventral appendage development during development of the adult fly, where they specify medial to distal appendage development. Cells lacking the disco genes cannot contribute to the medial and distal portions of ventral appendages. Further, ectopic disco transforms dorsal appendages toward ventral fates; in wing discs, the medial and distal leg development pathways are activated. Interestingly, this appendage role is conserved in the red flour beetle, Tribolium (where legs develop during embryogenesis), yet in the beetle we found no evidence for a head segmentation role. The lack of an embryonic head specification role in Tribolium could be interpreted as a loss of the head segmentation function in Tribolium or gain of this function during evolution of flies. However, we suggest an alternative explanation. We propose that the disco genes always function as appendage factors, but their appendage nature is masked during Drosophila embryogenesis due to the reduction of limb fields in the maggot style Drosophila larva.

XTsh3 is an essential enhancing factor of canonical Wnt signaling in Xenopus axial determination

In Xenopus, an asymmetric distribution of Wnt activity that follows cortical rotation in the fertilized egg leads to the dorsal-ventral (DV) axis establishment. However, how a clear DV polarity develops from the initial difference in Wnt activity still remains elusive. We report here that the Teashirt-class Zn-finger factor XTsh3 plays an essential role in dorsal determination by enhancing canonical Wnt signaling. Knockdown of the XTsh3 function causes ventralization in the Xenopus embryo. Both in vivo and in vitro studies show that XTsh3 substantially enhances Wnt signaling activity in a beta-catenin-dependent manner. XTsh3 cooperatively promotes the formation of a secondary axis on the ventral side when combined with weak Wnt activity, whereas XTsh3 alone has little axis-inducing ability. Furthermore, Wnt1 requires XTsh3 for its dorsalizing activity in vivo. Immunostaining and protein analyses indicate that XTsh3 is a nuclear protein that physically associates with beta-catenin and efficiently increases the level of beta-catenin in the nucleus. We discuss the role of XTsh3 as an essential amplifying factor of canonical Wnt signaling in embryonic dorsal determination.

Expanded and fat regulate growth and differentiation in the Drosophila eye through multiple signaling pathways

Mutations in the expanded gene act as hyperplastic tumor suppressors, interfere with cell competition and elevate Dpp signaling. Unlike Dpp overexpression, ex causes few patterning defects. Our data suggest that patterning effects are partly masked by antagonistic roles of other signaling pathways that are also activated. ex causes proliferation of cells in the posterior eye disc that are normally postmitotic. ex mutations elevate Wg signaling, but Dpp signaling antagonizes patterning effects of Wg. By contrast, if Dpp signaling is blocked in ex mutant cells, the elevated Wg signaling preserves an immature developmental state and prevents retinal differentiation. An effect of ex mutations on vesicle transport is suggested by evidence for altered sterol distribution. Mutations in ft show effects on proliferation, Wg signaling and sterols very similar to those of ex mutations. During disc growth, ex was largely epistatic to ft, and the Warts pathway mutation hippo largely epistatic to ex. Our data suggest that ft and ex act partially through the Warts pathway.

Growth control in the proliferative region of the Drosophila eye–head primordium: The elbow–noc gene complex

Notch signaling is involved in cell differentiation and patterning, as well as in the regulation of growth and cell survival. Notch activation at the dorsal-ventral boundary of the Drosophila eye-head primordium leads to the expression of the secreted protein Unpaired, a ligand of the JAK-STAT pathway that induces cell proliferation in the undifferentiated tissue. The zinc finger proteins encoded by elbow and no ocelli are expressed in the highly proliferative region of the eye-head primordium. Loss of elbow and no ocelli activities induces overgrowths of the head capsule, without inducing Upd expression de novo. These overgrowths depend on Notch activity suggesting that elbow and noc repress a Upd independent role of Notch in driving cell proliferation. When the size of the overgrown tissue is increased, ectopic antenna and eye structures can be found. Thus, tight regulation of the size of the eye-head primordium by elbow and no ocelli is crucial for proper fate specification and generation of the adult structures.

Teashirt 3 expression in the chick embryo reveals a remarkable association with tendon development

Drosophila teashirt (tsh) is involved in the patterning of the trunk identity together with the Hox genes. In addition, it is also a player in the Wingless and the Hedgehog pathways. In birds and mammals, three Tshz genes are identified and the expression patterns for mouse Tshz1 and Tshz2 have been reported during embryogenesis. Recently, we showed that all three mouse Tshz genes can rescue the Drosophila tsh loss-of-function phenotype, indicating that the function of the teashirt genes has been conserved during evolution. Here we describe the expression pattern of chick TSHZ3 during embryogenesis. Chick TSHZ3 is expressed in several tissues including mesodermal derivatives, the central and peripheral nervous systems. Emphasis is laid on the dynamic expression occurring in regions of the somites and limbs where tendons develop. We show that TSHZ3 is activated in the somites by FGF8, a known inducer of the tendon marker SCX.

Fate map of the distal portion of Drosophila proboscis as inferred from the expression and mutations of basic patterning genes

The late-third-instar labial disc is comprised of two disc-proper cell layers, one representing mainly the ventral half of the anterior compartment (L-layer) and the other, the dorsal half of the anterior compartment and most, if not all, of the posterior compartment (M-layer). In the L-layer, Distal-less represses homothorax whereas no Distal-less-dependent homothorax repression occurs in the M-layer where Distal-less is coexpressed with homothorax. In wild-type labial discs, clawless, one of the two homeobox genes expressed in distal cells receiving maximum (Decapentaplegic+Wingless) signaling activity in leg and antennal discs, is specifically repressed by proboscipedia. A fate map, inferred from data on basic patterning gene expression in larval and pupal stages and mutant phenotypes, indicates the inner surface of the labial palpus, which includes the pseudotracheal region, to be a derivative of the distal portion of the M-layer expressing wingless, patched, Distal-less and homothorax. The outer surface of the labial palpus with more than 30 taste bristles derives from an L-layer area consisting of dorsal portions of the anterior and posterior compartments, each expressing Distal-less. Our analysis also indicates that, in adults and pupae, the anterior-posterior boundary, dividing roughly equally the outer surface of the distiproboscis, runs along the outer circumference of the inner surface of distiproboscis.

Distinct functions of homeodomain-containing and homeodomain-less isoforms encoded by homothorax

The homothorax (hth) gene of Drosophila melanogaster is required for executing Hox functions, for head development, and for forming the proximodistal (PD) axis of the appendages. We show that alternative splicing of hth generates two types of protein isoforms, one that contains a DNA-binding homeodomain (HthFL) and one that does not contain a homeodomain (HDless). Both types of Hth isoforms include the evolutionarily conserved HM domain, which mediates a direct interaction with Extradenticle (Exd), another homeodomain protein. We show that although both HthFL and HDless isoforms of Hth can induce the nuclear localization of Exd, they carry out distinct sets of functions during development. Surprisingly, we find that many of hth's functions, including PD patterning and most Hox-related activities, can be executed by the HDless isoforms. In contrast, antennal development shows an absolute dependency on the HthFL isoform. Thus, alternative splicing of hth results in the generation of multiple transcription factors that execute unique functions in vivo. We further demonstrate that the mouse ortholog of hth, Meis1, also encodes a HDless isoform, suggesting that homeodomain-less variants of this gene family are evolutionarily ancient.

jing is required for wing development and to establish the proximo-distal axis of the leg in Drosophila melanogaster

The establishment of the proximo-distal (PD) axis in the legs of Drosophila melanogaster requires the expression of a nested set of transcription factors that are activated in discreet domains by secreted signaling molecules. The precise regulation of these transcription factor domains is critical for generating the stereotyped morphological characteristics that exist along the PD axis, such as the positioning of specific bristle types and leg joints. Here we provide evidence that the Zn-finger protein encoded by the gene jing is critical for PD axis formation in the Drosophila legs. Our data suggest that jing represses transcription and that it is necessary to keep the proximal gene homothorax (hth) repressed in the medial domain of the PD axis. We further show that jing is also required for alula and vein development in the adult wing. In the wing, Jing is required to repress another proximal gene, teashirt (tsh), in a small domain that will give rise to the alula. Interestingly, we also demonstrate that two other genes affecting alula development, Alula and elbow, also exhibit tsh derepression in the same region of the wing disc as jing- clones. Finally, we show that jing genetically interacts with several members of the Polycomb (Pc) group of genes during development. Together, our data suggest that jing encodes a transcriptional repressor that may participate in a subset of Pc-dependent activities during Drosophila appendage development.

Requirement of Abdominal-A and Abdominal-B in the developing genitalia of Drosophila breaks the posterior downregulation rule

The genitalia of Drosophila derive from the genital disc and require the activity of the Abdominal-B (Abd-B) Hox gene. This gene encodes two different proteins, Abd-B M and Abd-B R. We show here that the embryonic genital disc, like the larval genital disc, is formed by cells from the eighth (A8), ninth (A9) and tenth (A10) abdominal segments, which most likely express the Abd-B M, Abd-B R and Caudal products, respectively. Abd-B m is needed for the development of A8 derivatives such as the external and internal female genitalia, the latter also requiring abdominal-A (abd-A), whereas Abd-B r shapes male genitalia (A9 in males). Although Abd-B r represses Abd-B m in the embryo, in at least part of the male A9 such regulation does not occur. In the male A9, some Abd-B m(-)r(-) or Abd-B r(-) clones activate Distal-less and transform part of the genitalia into leg or antenna. In the female A8, many Abd-B m(-)r(-) mutant clones produce similar effects, and also downregulate or eliminate abdominal-A expression. By contrast, although Abd-B m is the main or only Abd-B transcript present in the female A8, Abd-B m(-) clones induced in this primordium do not alter Distal-less or abd-A expression, and transform the A8 segment into the A4. The relationship between Abd-B and abd-A in the female genital disc is opposite to that of the embryonic epidermis, and contravenes the rule that posteriorly expressed Hox genes downregulate more anterior ones.

Restricted teashirt expression confers eye-specific responsiveness to Dpp and Wg signals during eye specification in Drosophila

In Drosophila, the eye primordium is specified as a subdomain of the larval eye disc. Here, we show that the Zn-finger transcription factor teashirt (tsh) marks the region of the early eye disc where the eye primordium will form. Moreover, tsh misexpression directs eye primordium formation in disc regions normally destined to form head capsule, something the eye selector genes eyeless (ey) and twin of eyeless (toy) are unable to do on their own. We present evidence that tsh induces eye specification, at least in part, by allowing the activation of eye specification genes by the wingless (wg) and decapentaplegic (dpp) signaling pathways. Under these conditions, though, terminal eye differentiation proceeds only if tsh expression is transient.

Insect appendages and comparative ontogenetics

It is arguable that the evolutionary and ecological success of insects is due in large part to the versatility of their articulated appendages. Recent advances in our understanding of appendage development in Drosophila melanogaster, as well as functional and expression studies in other insect species have begun to frame the general themes of appendage development in the insects. Here, we review current studies that provide for a comparison of limb developmental mechanisms acting at five levels: (1) the specification of ventral appendage primordia; (2) specification of the limb axes; (3) regulation and interactions of genes expressed in specific domains of the proximal-distal axis, such as Distal-less; (4) the specification of appendage identity; and (5) genetic regulation of appendage allometry.

Parallels between the proximal-distal development of vertebrate and arthropod appendages: homology without an ancestor?

Evolutionary studies suggest that the limbs of vertebrates and the appendages of arthropods do not share a common origin. However, recent genetic studies show new similarities in their developmental programmes. These similarities might be caused by the independent recruitment of homologous genes for similar functions or by the conservation of an ancestral proximal-distal development programme. This basic programme might have arisen in an ancestral outgrowth and been independently co-opted in vertebrate and arthropod appendages. It has subsequently diverged in both phyla to fine-pattern the limb and to control phylum-specific cellular events. We suggest that although vertebrate limbs and arthropod appendages are not strictly homologous structures they retain remnants of a common ancestral developmental programme.

Activation of Rac1 or Cdc42 during early morphogenesis of eye discs induces ectopic antennae in Drosophila

The Rho family small guanosine triphosphatases (GTPases) play important roles in many cellular processes, especially in regulation of cytoskeletal organization. In this study, I examined the functions of Rac1 and Cdc42 for disc morphogenesis in Drosophila. I expressed either a constitutively active form or a dominant negative form of each protein during early morphogenesis of eye discs. Inactivation of Rac1 or Cdc42 resulted in small eye phenotypes. On the other hand, I found that activation of either Rac1 or Cdc42 induces ectopic antennae. In some cases, an almost complete antenna was observed instead of an eye, which was possibly transformation from an eye to an antenna. As a molecular evidence for the ectopic antennae, I found that the Distal-less protein, which is essential for the distalization process, was ectopically induced in the eye discs. I also found that the Decapentaplegic and Wingless proteins, which are upstream morphogenetic signaling proteins of the distalization process, could be ectopically induced by activation of Rac1 or Cdc42. My observations suggest novel functions of Rac1 and Cdc42 for disc morphogenesis.

Diverse developmental mechanisms contribute to different levels of diversity in horned beetles

An ongoing challenge to evolutionary developmental biology is to understand how developmental evolution on the level of populations and closely related species relates to macroevolutionary transformations and the origin of morphological novelties. Here we explore the developmental basis of beetle horns, a morphological novelty that exhibits remarkable diversity on a variety of levels. In this study, we examined two congeneric Onthophagus species in which males develop into alternative horned and hornless morphs and different sexes express marked sexual dimorphism. In addition, both species differ in the body region (head vs. thorax) that develops the horn. Using a comparative morphological approach we show that prepupal growth of horn primordia during late larval development, as well as reabsorption of horn primordia during the pupal stage, contribute to horn expression in adults. We also show that variable combinations of both mechanisms are employed during development to modify horn expression of different horns in the same individual, the same horn in different sexes, and different horns in different species. We then examine expression patterns of two transcription factors, Distal-less (Dll) and aristaless (al), in the context of prepupal horn growth in alternative male morphs and sexual dimorphisms in the same two species. Expression patterns are qualitatively consistent with the hypothesis that both transcription factors function in the context of horn development similar to their known roles in patterning a wide variety of arthropod appendages. Our results suggest that the origin of morphological novelties, such as beetle horns, rests, at least in part, on the redeployment of already existing developmental mechanisms, such as appendage patterning processes. Our results also suggest, however, that little to no phylogenetic distance is needed for the evolution of very different modifier mechanisms that allow for substantial modulation of trait expression at different time points during development in different species, sexes, or tissue regions of the same individual. We discuss the implications of our results for our understanding of the evolution of horned beetle diversity and the origin and diversification of morphological novelties.

Functional analyses of tiptop and antennapedia in the embryonic development of Oncopeltus fasciatus suggests an evolutionary pathway from ground state to insect legs

In insects, selector genes are thought to modify the development of a default, or 'ground state', appendage into a tagma-specific appendage such as a mouthpart, antenna or leg. In the best described example, Drosophila melanogaster, the primary determination of leg identity is thought to result from regulatory interactions between the Hox genes and the antennal-specifying gene homothorax. Based on RNA-interference, a functional analysis of the selector gene tiptop and the Hox gene Antennapedia in Oncopeltus fasciatus embryogenesis is presented. It is shown that, in O. fasciatus, tiptop is required for the segmentation of distal leg segments and is required to specify the identity of the leg. The distal portions of legs with reduced tiptop develop like antennae. Thus, tiptop can act as a regulatory switch that chooses between antennal and leg identity. By contrast, Antennapedia does not act as a switch between leg and antennal identity. This observation suggests a significant difference in the mechanism of leg specification between O. fasciatus and D. melanogaster. These observations also suggest a significant plasticity in the mechanism of leg specification during insect evolution that is greater than would have been expected based on strictly morphological or molecular comparisons. Finally, it is proposed that a tiptop-like activity is a likely component of an ancestral leg specification mechanism. Incorporating a tiptop-like activity into a model of the leg-specification mechanism explains several mutant phenotypes, previously described in D. melanogaster, and suggests a mechanism for the evolution of legs from a ground state.

Differing strategies for the establishment and maintenance ofteashirtandhomothoraxrepression in theDrosophilawing

Secreted signaling molecules such as Wingless (Wg) and Decapentaplegic(Dpp) organize positional information along the proximodistal (PD) axis of the Drosophila wing imaginal disc. Responding cells activate different downstream targets depending on the combination and level of these signals and other factors present at the time of signal transduction. Two such factors, teashirt (tsh) and homothorax (hth), are initially co-expressed throughout the entire wing disc, but are later repressed in distal cells, permitting the subsequent elaboration of distal fates. Control of tsh and hth repression is, therefore,crucial for wing development, and plays a role in shaping and sizing the adult appendage. Although both Wg and Dpp participate in this control, their specific contributions remain unclear. In this report, we analyze tshand hthregulation in the wing disc, and show that Wg and Dpp act independently as the primary signals for the repression of tsh and hth, respectively. In cells that receive low levels of Dpp, hth repression also requires Vestigial (Vg). Furthermore, although Dpp is required continuously for hth repression throughout development, Wg is only required for the initiation of tshrepression. Instead, the maintenance of tsh repression requires Polycomb group (PcG) mediated gene silencing, which is dispensable for hth repression. Thus, despite their overall similar expression patterns, tsh and hth repression in the wing disc is controlled by two very different mechanisms.

A mosaic genetic screen reveals distinct roles for trithorax and polycomb group genes in Drosophila eye development

The wave of differentiation that traverses the Drosophila eye disc requires rapid transitions in gene expression that are controlled by a number of signaling molecules also required in other developmental processes. We have used a mosaic genetic screen to systematically identify autosomal genes required for the normal pattern of photoreceptor differentiation, independent of their requirements for viability. In addition to genes known to be important for eye development and to known and novel components of the Hedgehog, Decapentaplegic, Wingless, Epidermal growth factor receptor, and Notch signaling pathways, we identified several members of the Polycomb and trithorax classes of genes encoding general transcriptional regulators. Mutations in these genes disrupt the transitions between zones along the anterior-posterior axis of the eye disc that express different combinations of transcription factors. Different trithorax group genes have very different mutant phenotypes, indicating that target genes differ in their requirements for chromatin remodeling, histone modification, and coactivation factors.